1. Field of the Invention
The present invention is directed to a process for the production of heat curable, flexible, polyurethane molded foams and to the molded foams produced therefrom.
2. Description of the Prior Art
It is well known that heat curable, flexible, polyurethane molded foams are widely used, inter alia, in the construction of motor vehicles. The quality of the flexible polyurethane molded foams conventionally used for seats and back rests depends to a large extent upon their gross density. It has been found in practice that regardless of the nature of the basic raw materials (polyols/isocyanates) used for the production of the heat molded foams, a certain level of gross density is required for certain applications (seat, bench, back rest and required for meeting the requirements of these particular applications, especially in regard to the hard wearing properties in use.
The gross density obtained is primarily determined by the water content of the mixture of starting materials. The quantities of water used are typically up to 5 parts by weight (more typically about 2.5 to 3.5 parts by weight) per 100 parts by weight of polyol, depending upon the level of gross density required.
It has been found that the low gross densities desired in molded foams for the purpose of reducing weight and cost can only in exceptional cases be realized by increasing the water content beyond the quantities indicated. The reason for this is that as the water content increases (&gt;3.5 parts by weight per 100 parts by weight of polyol), correct catalytic adjustment of the chemical reactions (polymer formation/gas formation) which is essential for the foaming process becomes more difficult and the range of allowable operating conditions becomes greatly restricted. Apart from problems relating to the nature of the skin (molding temperature/mold release agent) and the stability of the foam, the range allowed for the isocyanate index also becomes narrower. It is for these reasons that any reduction in the gross density required or desired is preferably brought about by means of physical blowing agents (e.g. halogenated hydrocarbons). This method is state of the art and applies in principle to all heat curable molded foams but is not to be recommended on ecological grounds and for reasons of cost.
It has now surprisingly been found that in addition to the usual range of 1 to 5 parts by weight of water, atypically large quantities of water, i.e., more than 5.0 parts by weight per 100 parts by weight of the polyol component, may readily be used with isocyanates, preferably of the toluylene diisocyanate type. As a result, gross densities as low as 15 kg/m.sup.3 can be obtained in heat curable, flexible, polyurethane molded foams if carbonic acid diamide, preferably in a quantity of less than 1.0 part by weight, based on 100 parts by weight of the polyol component, and small quantities, preferably about 0.05 to 0.25 parts by weight, of an aminic co-catalyst are used and if, as a departure from the conventional practice of using an isocyanate index of 93 to 105, an index of less than 90, which is normally regarded as prohibitively low, is used for the foaming process.
Flexible polyurethane foams prepared with the addition of carbonic acid diamide have already been described in U.S. Pat. No. 3,479,304, but the required quantity of carbonic acid diamide is 1 to 5 parts by weight, based on 100 parts by weight of the polyol component. Water is used as the blowing agent in a quantity of 1 to 5 parts by weight, based on 100 parts by weight of the polyol component. In contrast to the process described in the present application, the process according to the U.S. Pat. in all cases uses halogenated isocyanates based on toluylene diisocyanate. The flexible polyurethane molded foams prepared according to the present invention require an aminic co-catalyst, preferably a tertiary amine, in addition to the carbonic acid diamide described above and the organic tin catalyst conventionally used for producing heat curable molded foams. The combination of an organic tin catalyst with a tertiary amine, however, is excluded in principle in the U.S. Pat.
The range for the isocyanate index set forth in the U.S. Pat. is from 90 to 120, in contrast to the process of the present invention which requires an index below 90. In addition, the process employed in the U.S. patent is directed to foaming in open molds instead of foaming inside closed molds in accordance with the present invention. The conditions used for foaming in closed molds and the physical properties of the resulting polyurethane foams differ substantially from the process of foaming in open molds and the products (free foam) obtained therefrom. The described after-heating of the foams is not required in accordance with the present invention.